1. Field of the Invention
The present invention relates to a coordinate inputting device, and, more particularly, to a coordinate inputting device in which the vibrations impartedto a vibration transmitting plate by a vibration pen are detected by a plurality of vibration sensors provided on the vibration transmitting plate so as to enable the coordinates representing a vibration input point to be detected using the time it takes for the vibrations to propagate on the vibration transmitting plate.
2. Related Background Art
Coordinate inputting devices for inputting manually written characters or manually drawn figures to a processing device such as a computer by utilizing various types of input pens and tablets are known in the art. In this type of information inputting method, image information composed of input characters or figures are output to a display such as a CRT display or to a recording device such as a printer.
In the above-described types of devices, coordinates on the tablet are conventionally detected, for example, according to the following methods:
(1) by detecting variations in the resistance of a sheet disposed so as to face a resistive film;
(2) by detecting electromagnetic or electrostatic induction of the opposing conductive sheets; or
(3) by detecting ultrasonic vibrations transmitted from an input pen to a tablet.
The above-described methods (1) and (2) have the disadvantage that use of a resistive film or conductive film precludes formation of a transparent tablet. On the other hand, according to method (3), a transparent material such as an acrylic plastic or glass plate can be used as the tablet, enabling the tablet to be superimposed on a display such as a liquid crystal display so that images can be written thereon as if they were being written on a sheet of paper.
Although coordinate detection with ultrasonic waves has the above-described advantage over the other detection methods, it suffers from the following problem:
In order to make use of the transparency of a tablet, the vibration transmitting plate which constitutes the tablet is made of a non-conductive material such as acrylic plastic or glass plate while the vibration sensor employed is a piezoelectric element which is a ceramic interposed between electrodes. In that case, various kinds of noise, such as electrostatically induced noises, enter the electrodes of the sensors, diminishing the accuracy at which the coordinates are detected.